This is a 371 filing of PCT/EP98/03879 filed Jun. 25, 1998.
The present invention relates to a transparent coating composition based on a sheet silicate, to its preparation and to its use for reducing the soiling tendency of facades.
Facades, like any other surface, become soiled over time owing to deposits from the air onto the surface. This soiling is manifested in graying. Using light microscopy and SEM investigations on facade coatings which have been weathered under open-air conditions it has been found that the soiling consists essentially of inorganic particles with a size of up to 10 xcexcm along with small fractions of soot.
U.S. Pat. No. 2,877,142 discloses the dirt repelling action of inorganic sols, such as silica sols, on surfaces. The use of silica sols on facades is hindered by their low viscosity. In the DIN 4 mm flow cup, these sols have a viscosity of 14.3 s (23xc2x0 C.). Application techniques used are brushing, rolling and spraying. When applied at a rate appropriate to that in practice of about 50-250 g/m2, preferably 50-150 g/m2, the silica sols run on vertical faces such as facades. It is therefore impossible to obtain the even coat thickness that is required for the coating to have a uniform dirt repelling effect. At application rates of from 50 g/m2 to 250 g/m2, splashes and water runs occur which prevent an even coating. A thixotropic or at least pseudoplastic consistency, which permits brushing tools to be used without problems, is not possible with sols. Such a consistency is obtained if in the DIN 4 mm flow cup efflux times of more than 100 s at least are measured.
The object of the present invention, then, is to provide a transparent, readily processable, color-neutral and at least pseudoplastic coating composition which permits an even application to facades and at the same time reduces their soiling tendency and the attenuation of gloss.
It has been found that only a few selected materials are compatible with this object for the coating composition of the invention. The choice of a suitable rheological material has a critical influence on the effect of the coating material. For instance, organic gel formers such as cellulose ethers, polyacrylates and polyurethane thickeners remove the activity of the silica sols in reducing soiling. In the case of inorganic gel formers, the soiling-reducing effect of the silica sols is not removed. For instance, with coating compositions based on silica sol and synthetic or natural sheet silicates a marked reduction was found in the soiling tendency. However, natural inorganic sheet silicates are unsuited to the set object owing to the lack of color neutrality of the coating composition. The observed yellowishness of the coating composition did not permit color-neutral drying. The yellowishness originates from the iron inclusions in the sheet silicates. Sheet silicates from all known deposit regions have iron inclusions. In addition, the natural sheet silicates are not completely transparent. The average particle sizes of the known natural sheet silicates are within the micrometer range. Pyrogenic silicas are likewise inorganic compounds, but cannot be processed with silica sol and water to give a homogeneous coating composition and so do not meet the set object. Precipitated silicas, again, are inorganic compounds, but have average particle sizes of more than one micrometer. Consequently, the coating composition prepared from them using silica sol and water does not dry to a transparent film.
It has surprisingly been found that synthetic inorganic sheet silicates which give a transparent gelatinous paste with water possess both per se and in combination with silica sols, a dirt repelling action and also comply with the other conditions of the set object. They are therefore suitable as coating compositions for the stated set object. The viscosity in this case is preferably established so that an even application of from 50 g/m2 to 250 g/m2 is possible. The consistency is raised to such an extent that in the DIN 4 mm flow cup efflux times of more than 100 s at least are measured. It proves even more favorable if the medium can no longer be found to have any fluidity. The resulting coating material shows thixotropic properties. Application to substrates can now be made, even using brushing tools, without splashes or runs. The resulting coating is transparent and does not show any color changes of the substrate.
The invention accordingly provides a coating composition comprising at least one sheet silicate which forms a colloidal gel in water. The coating composition is transparent and the sheet silicate used is preferably a nanoscale silicate having an average particle size of from 5 to 800 nm, preferably from 25 to 500 nm, in particular from 100 to 400 nm.
Sheet silicates suitable in accordance with the invention are available commercially; suitable examples include SKSxe2x80x9420/Saponite (Hoechst AG, Frankfurt, Germany), SKSxe2x80x9421/Hectorite (Hoechst AG; Frankfurt, Germany), Optigel(copyright) SH (Sud Chemie AG, Munich, Germany) and Laponite(copyright) RD (Deutsche Solvay Werke GmbH, Solingen, Germany). Mixtures of these sheet silicates with one another are also suitable. All of these colloidal gels, which have been prepared from water with synthetic inorganic sheet silicates, preferably having a particle size of below 1 micrometer, meet the object of the invention. The transparent coating composition also features the desired properties when other, at least colloidally soluble inorganic substances (for example, silica sols) are present.
In the text below, the invention is illustrated with reference to examples.
Various coating compositions were prepared and their action was tested on the substrate and in the soiling test. The coating compositions contained different gel-forming substances along with silica sols and water. The preparation and testing are described in detail.